A subsea well typically has a wellhead housing located on the sea floor. One or more casing hangers are supported in the wellhead housing, each located at the upper end of a string of casing. In one type of wellhead assembly, a tubing hanger located at the upper end of a string of tubing is installed in the wellhead housing. The operator may perforate the well at that point and install a wire line plug in the production passage of the tubing hanger. The operator then lands a production tree on the wellhead housing, the tree having a number of valves for controlling the well fluid. The tree has a production flow passage and an isolation sub that stabs into the production passage of the tubing hanger. The operator then removes the wire line plug by lowering a tool through the production flow passage of the tree. For a workover operation involving pulling of the tubing hanger, the tree must be disconnected from the wellhead housing. If the tree needed to be retrieved for repair work, this can be done without pulling the tubing.
In another type of wellhead assembly, the tree is installed on the wellhead housing before running the tubing. The operator connects the drilling riser to the tree, lowers the tubing hanger through the drilling riser and lands the tubing hanger in the tree. The tubing hanger has a lateral flow outlet that registers with a lateral flow outlet in the tree. The operator installs a wire line plug in the tubing hanger vertical bore above the flow outlet. The tree does not need to be disconnected from the wellhead housing for pulling the tubing for a workover operation. If the tree needed to be retrieved for repair, the tubing would have to be pulled.
U.S. Pat. No. 5,372,199 discloses a configuration with a lower tubing hanger landed in the wellhead housing and supporting a string of tubing. A tree having a lateral flow outlet lands on the wellhead housing. An upper tubing hanger is landed in the bore of the tree. The upper tubing hanger has an isolation tube on its lower end that stabs into engagement with the production passage in the lower tubing hanger. The upper tubing hanger has a lateral flow outlet that registers with the lateral flow outlet of the tree. An internal tree cap is installed within the bore of the tree above the upper tubing hanger. The tubing can be pulled without disconnecting the tree from the wellhead housing by first retrieving the tree cap, then the upper tubing hanger, and then the lower tubing hanger and tubing. Similarly, the tree can be retrieved without pulling the tubing.
In the various configurations described above, the tree is a large, heavy and complex assembly that is run on a string of drill pipe. The running procedure requires a vessel with a derrick. It may not be economical to utilize the same vessel that drilled the well to complete the well and install the tree. Designs for trees that can be run on a lift line are known, but these systems typically do not have the ability to pull the tubing without disturbing the connection between the tree and the wellhead housing.